1. Field of the Invention
The present invention relates to exposure apparatus and device manufacturing methods, and more particularly to an exposure apparatus that is used in a lithographic process when manufacturing electronic devices such as a semiconductor or a liquid crystal display device, and a device manufacturing method that uses such an exposure apparatus.
2. Description of the Related Art
In a lithographic process to produce electronic devices such as a semiconductor (integrated circuit) or a liquid crystal display device, projection exposure apparatus are used that transfer an image of a pattern of a mask or a reticle (hereinafter generally referred to as a ‘reticle’) via a projection optical system onto shot areas on a wafer coated with a resist (photosensitive agent) or a photosensitive substrate such as a glass plate (hereinafter referred to as a ‘glass plate’ or a ‘wafer’). Conventionally, the reduction projection exposure apparatus based on a step-and-repeat method (the so-called stepper) has been widely used as such a projection exposure apparatus; however, recently, the type of projection exposure apparatus that performs exposure while the reticle and the wafer are synchronously scanned (the so-called scanning stepper) is also drawing attention.
The resolution of the projection optical system installed in the projection exposure apparatus becomes higher when the exposure wavelength used is shorter or when the numerical aperture (NA) of the projection optical system is higher. Therefore, in order to cope with finer integrated circuits, the exposure wavelength used in a projection exposure apparatus is becoming shorter year by year and the numerical aperture of the projection optical system becoming higher. The wavelength widely used for exposure at present is 248 nm generated by the KrF excimer laser; however, the wavelength generated by the ArF excimer laser, 193 nm, which is shorter, has also been put to practical use.
In addition, when exposure is performed, depth of focus (DOF) is also equally important as resolution. Resolution R and depth of focus δ can be expressed as the following equations.R=k1·λ/NAδ=k2·λ/NA2 
In this case, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. From equations (1) and (2), it can be seen that when exposure wavelength λ is shortened and numerical aperture NA increased for a higher resolution R, depth of focus δ becomes narrower. In a projection exposure apparatus, however, because exposure is performed in an auto-focus method where the surface of the wafer is adjusted so that it matches the image plane of the projection optical system, a wide depth of focus δ is preferable to some extent. Therefore, proposals for substantially widening the depth of focus have been made in the past, such as the phase shift reticle method, the modified illumination method, and the multiplayer resist method.
As is described above, in the conventional projection exposure apparatus, depth of focus is becoming narrower due to shorter wavelength of the exposure light and larger numerical aperture of the projection optical system. And, in order to cope with higher integration of the integrated circuits, it is certain that the exposure wavelength will become much shorter in the future; however, in such a case, the depth of focus may become too narrow so that there may not be enough margin during the exposure operations.
Accordingly, a proposal on an immersion method has been made as a method for substantially shortening the exposure wavelength while enlarging (widening) the depth of focus more than the depth of focus in the air. In this immersion method, resolution is improved by filling a space between the lower surface of the projection optical system and the surface of the wafer with liquid such as water or an organic solvent to make use of the fact that the wavelength of the exposure light in the liquid becomes 1/n of the wavelength in the air (n is the refractive index of the liquid which is normally around 1.2 to 1.6). In addition, when this immersion method is applied, the depth of focus is substantially enlarged n times when comparing it with the case when the same resolution is obtained without applying the immersion method to the projection optical system (supposing that such a projection optical system can be made). That is, the depth of focus is enlarged n times than in the atmosphere (for example, refer to the pamphlet of International Publication Number WO99/49504 or the like).
According to the projection exposure method and the apparatus disclosed in International Publication Number WO99/49504 referred to above (hereinafter referred to as ‘conventional art’), the immersion method allows exposure to be performed with high resolution as well as a greater depth of focus than in the air, and also allows the liquid to be filled stably between the projection optical system and the substrate, that is, allows the liquid to be held, even when the projection optical system and the wafer are relatively moved.
In the conventional art, however, it was difficult to recover the liquid completely, and it was highly probable for the liquid used for immersion to remain on the wafer. In such a case, the heat of vaporization when the remaining liquid vaporizes was likely to cause a temperature distribution or a refractive index change in the atmosphere, and such phenomena could cause a measurement error in the laser interferometer used for measuring the position of the stage on which the wafer is mounted. Furthermore, the remaining liquid on the wafer could flow to the back of the wafer, making the wafer stick to the carrier arm and difficult to separate. In addition, the gas (air) flow of the atmosphere around the liquid could be distorted with the liquid-recovery operation, which could cause a temperature distribution or a refractive index change in the atmosphere.
In addition, in the conventional art, when exposing an edge shot on the wafer, in the case the projection area of the projection optical system was located near the edge of the wafer, the liquid could leak outside the wafer which would interfere with the favorable image forming of the projected image of the pattern. Furthermore, when the wafer was not available underneath the projection optical system, it was difficult to hold the liquid referred to above; therefore, when exposure was to begin after wafer exchange on a wafer that has been exchanged, the start had to be delayed until the wafer was moved under the projection optical system and the liquid has been supplied to a space between the projection optical system and the wafer.
In addition, peripheral units such as a sensor like a focus sensor or an alignment sensor have to be arranged in the vicinity of the projection optical system. In the conventional art, however, because the supply piping, the recovery piping, and the like were arranged on the outer side of the projection optical system, the degree of freedom was limited when such peripheral units were disposed.
In addition, in the conventional art, bubbles could be found or formed in the supplied liquid, and when such bubbles come in the space between the projection optical system and the substrate, not only did they decrease the transmittance of the exposure light and cause uneven exposure but could also cause defective imaging when the projected image of the pattern is formed.
Furthermore, because the exposure light irradiates the liquid between the projection optical system and the substrate, a temperature change (a refractive index change) could occur in the liquid, which may degrade the imaging quality of the pattern. In addition, the pressure of the liquid between the projection optical system and the substrate may cause the wafer stage that holds the projection optical system and the wafer to vibrate or to incline, which would degrade the transfer accuracy of the pattern onto the wafer. Moreover, when the liquid flows with respect to the projection optical system in the projection area of the pattern, temperature inclination or pressure inclination related to the direction of the flow may occur, which may be the cause of aberration of the projection optical system such as inclination of the image plane or the cause of partial degrading in transfer accuracy of the pattern, which deteriorates the line width uniformity of the transferred image of the pattern.
Accordingly, various improvements can be made to the examples of the conventional art referred to above.